Imagine There’s No God Particle

The LHC is back in business after a technical stop, getting ready to collide protons for the next couple months, perhaps reaching an integrated luminosity of about 5 inverse femtobarns. This is a factor of four higher than the luminosity used in most analyses that have been made public so far, and the latest projections are that this should allow an exclusion of a Higgs over the entire expected mass range at 95% confidence level, if such a particle really doesn’t exist.

My pre-LHC predictions (see here) of five years ago have held up well, and nothing yet has changed my view that a Higgs particle scenario and a no-Higgs scenario are equally likely. The best argument for a Higgs in the mass range of 114-145 GeV is that it’s the simplest way anyone has found of making the Standard Model work, and explains a range of precision electroweak measurements.

The best argument against the Higgs is that elementary linear scalar fields are problematic (since not asymptotically free) and esthetically displeasing (not geometrical and constrained by symmetries, so lead to lots of undetermined parameters, mainly for the Yukawas that determine the masses of all fermions). By analogy with the theory of superconductivity though, one can imagine that the Higgs makes a good low-energy effective theory (a la Landau-Ginzburg), even if there’s a more interesting fundamental theory, which may require going to a smaller distance scale (a la BCS theory). As the allowed Higgs mass range has narrowed though, I’m starting to think that there may be something to the argument that it’s implausible that the mass would end up being in the hardest mass range for colliders to examine. More likely it’s just not there, and the hardest range is the last one to fall to experiment.

By the way, I was interviewed about this on a Wired podcast (see here), not sure how it turned out. I don’t think I said anything surprising or controversial.

The imminent arrival of an experimental result deciding the issue of the SM Higgs has focused attention on what the implications will be, and here’s what I’ve been thinking:

If the SM Higgs is found, there will be rejoicing at first at CERN and within the physics community, and an appropriately proud announcement to the public. Debate will begin on who gets the Nobel: experimentalists? which of the 6000+ people at LHC/CMS/ATLAS? or theorists? Anderson/Higgs/Englert/Brout/Guralnik/Hagen/Kibble, or ? I gather Brout is no longer with us, maybe this will have to wait until the list gets down to three by attrition. Probably the best case would be for Weinberg/Salam, but they already were rewarded for the SM. Maybe the Swedes could make Weinberg’s a double. The LHC experimentalists would have an active research program for many years trying to measure the Higgs properties. Theorists though would face the gloomy prospect that these would just agree with the SM. We’d be stuck pretty much where we have been for thirty years: no clues as to how to do better than the SM.

What though if the SM Higgs gets ruled out? CERN may consider this an embarassment, but it’s actually a far more exciting result, one even more worthy of the Nobel than finding the long-sought particle. SUSY enthusiasts will claim this means it’s a SUSY Higgs, and model builders will get to work on constructing more complicated models designed to explain the result by making the Higgs even harder to see (Matt Strassler is starting to write about such models here). My guess would be though that no Higgs means the argument from esthetics was right, so adding in more scalar fields in some complex pattern isn’t a very plausible explanation of the null result.

A commenter here pointed out that this possibility was discussed during the debate over the SSC, when it was argued that, in the case of no Higgs, you would need a 40 TeV machine to look at W/Z scattering, to get information about what was really going on. The LHC should be capable of quite high luminosity, which may compensate for its lower energy in such searches, see a recent discussion here.

My own very vague favorite idea has always been that, non-perturbatively, there’s something important we’re missing in our understanding of gauge symmetry in chiral gauge theories and that this may hold the secret to the mystery of electroweak symmetry breaking. While this idea has been a motivation for research I’ve been pursuing in recent years, I can’t claim to have made any progress on it. My second real blog posting here was about this, back in 2004, leading to a torrent of abuse. Maybe if there’s no Higgs, SUSY and extra dimensions are gone, this could become a legitimate question in the eyes of mainstream theorists.

You-hoo-oo-oo-oo, you may say I’m a dreamer
But I’m not the only one…

Update: It seems that I’m definitely not the only one inspired by John Lennon recently, with CIP beating me to this a while ago.

Update: On the topic of this posting, see Slava Rychkov’s talk that just appeared on the arXiv. From the summary:

We have seen many impressive new physics limits set at this conference. But, have we ever truly believed in the models that are being pushed away? Z-prime, CMSSM, split SUSY, to name a few? I myself certainly never believed in these. Take Z-prime. In spite of what you may have heard, this is a completely unmotivated extension of the SM. It solves nothing of its problems and has nothing to do with Naturalness. Same for split SUSY, anathema to Naturalness. CMSSM is the only victim on the list for which I feel sorry, but we can’t give up on SUSY just because this straightjacketed version of it failed.

Another early casualty has been the Large Extra Dimensions scenario. But again, this was hardly a bona fide solution to the hierarchy problem. The mechanism which cuts off the Higgs mass quadratic divergence has not been concretely specified. It’s only because the idea was so original that we ever gave it the benefit of the doubt. Now with LHC limits on the (4+n)-dimensional Planck scale already a factor two above the Tevatron limits, it’s basically gone. The truth is, apart from SUSY, there are only two other motivated scenarios for TeV-scale physics: strong EWSB and Composite Higgs. I mentioned some of the signals expected in these models. Unlike CMSSM, they typically require much higher luminosity to be seen.

89 Responses to Imagine There’s No God Particle

“the vacuum energy contribution of the Higgs field results in a cosmological constant fifty times larger than the one actually measured,”

I think you mean about 10^{55} times larger than measured. The observed cc corresponds to a vacuum energy density $ \Lambda / (8 \pi G_N) $ of about (2.3 x 10^{-3} eV)^4, while the Higgs vev gives in order of magnitude about (100 GeV)^4.

I once read in an article (of which I don’t seem to remember the name or author(s)) that a pseudo-Riemannian metric or frame field may act as a Higgs field. The article dealt with the mechanics and its consistency with the electroweak Higgs mechanism. This, albeit just like a scalar Higgs field, would shed light to the problematicity and inelegance of elementary scalar fields unlike the scalar Higgs, and would explain the origin of the Higgs and why it occupies every point in space, since the metric/frame field is simply a property of space-time, and every point in space has a metric. Also when it is switched from Einstein frame to Jordan frame, the scalar Higgs mechanism is recovered. Thus this would be a more than decent argument to resort to whether the Higgs is found or not.

I’ve always had some fondness for the idea that the Higgs field has something to do with the choice of a time direction. But the problem with any of the many ideas around that interpret the Higgs field as some element of space-time geometry is that you need to come up with a real consistent and complete theory, i.e. at least a Lagrangian or something. You’ll need to explain where the Yukawa couplings come from, and thus you’ll explain all particle masses. Since you’re unifying a crucial element of the electroweak theory and the space-time geometry, set by gravity, you probably also need to figure out how to quantize gravity and unify it with the Standard Model….

Actually the decay width of the SM Higgs is only 10MeV for a Higgs mass < 135GeV and is much too narrow to be measured by the LHC. The very narrow width is governed by the dominant decay to quark anti-quark pairs in this energy range and is not related to precision electroweak fits, it's a theoretical calculation. The RPP reference I gave above has a nice plot of SM Higgs width vs mass on page 9 (figure 4). For a Higgs mass above 145GeV the width expands dramatically but ironically the LHC has ruled those masses out. If the Higgs exists in the range still open to it, the LHC is not well-placed to elucidate its properties; you really need a muon anti-muon collider to achieve the precision necessary to measure such narrow widths. Fermilab is planning to build one but will they get the money?

@null
Who knows? If the Tevatron shut down six months from now it would make little difference to the huge dataset they already have, since they have virtually maximized the data taking capability already, whereas the LHC needs to and is aggressively accumulating data and is at a disadvantage anyway in the lower end of the 115-145 GeV range. It's also not clear to me whether DZero and CDF are now pooling their resources compared to Atlas and CMS which, AFAIK, are still operating as independent competing teams. Morale might also be an issue at the Tevatron. And there is the complicating factor that quite a few researchers have a foot in both the Tevatron and LHC camps.

Do you really need a muon-anti-muon collider to measure the SM Higgs width? Why couldn’t it be done with the ILC or CLIC? Besides money issues, my understanding is that it’s still not at all clear if the technology for a muon-anti-muon collider (and detectors…) is even feasible. One surprising problem that comes up is a radiation hazard from so many neutrinos…

@DB
I think the point about the Higgs width was that hidden sector fields can have renormalisable couplings to the operator |H|^2, where H is the Higgs. This won’t
affect its production rate at a collider, but will certainly increase its width if it is kinematically allowed to decay to such hidden sector states.

Sure, DB, if the Higgs couples exclusively to the SM particles. But maybe, as Rhys points out, there are other as yet undiscovered fields that the Higgs couples to that broaden it. Maybe the Higgs is so broad that it is under the LHC backgrounds.

A limit plot that treated the Higgs width as an unknown parameter would be an interesting CMS/ATLAS output.

I think one of the well-known members of the I-missed-the-J/Psi club had a nice plot in their thesis that included resonance width… at that time no-one expected the *narrowness* of the J/Psi, and so scan points skipped over the resonance. But a proper limit was still made, where the limit admitted the possibility that a narrow resonance would have been passed over.

@Peter
Given a Higgs width in the 10 MeV range a muon antimuon collider is your only option. This is because the production of Higgs at high rates in the s-channel is a unique feature of muon colliders – whence the moniker “Higgs Factory”. (s-channel is the scattering mode that describes the gluon-gluon fusion leading to Higgs as an intermediate particle which then decays to a bottom antibottom quark pair – this largely dominates if Higgs is around 120GeV and unlike the LHC, it’s what the Tevatron is tuned to observe). Next, you have very small radiative losses (vs electron colliders) which allow you to achieve extremely narrow beam energy spreads. Plus, you can determine the beam energy very precisely using the time-dependent asymmetry from the polarized muons in the beam itself. Also, because the Higgs couples to particles based on their mass, an electron-positron collider will generate too few Higgs vs its much heavier cousin, the muon. In turn, this enhanced coupling of the Higgs boson to the muon makes for an ideal opportunity to perform ultra-high precision measurements of the Higgs mass. Finally, because of the greatly reduced radiative losses (bremsstrahlung) you can design circular muon colliders in lieu of the traditional linear ep colliders. It’s this unique combination of features that gives a muon collider its preferred status as a high-precision probe of low mass Higgs properties.
That the LHC would need help in measuring Higgs properties is no surprise. Electron-positron colliders have long been been the backbone of the high-precision measurements of Standard Model parameters, the LEP2 studies of the Z boson being a prime example. Lepton colliders are just better suited to high precision measurement than their hadron equivalents.
As to the high levels of neutrinos from decaying muons, this is a considerable bonus, because of the high luminosity of the resulting beam and the fact that its flavour content is precisely known makes it an excellent source for neutrino spectroscopy. In addition, it’s expected that muon decay within a storage ring can be greatly reduced using ionization cooling so that decay can be tuned as and when secondary neutrino experiments require. This is currently the object of study of the International Muon Ionization Cooling Experiment (MICE), which recently became operational and is due to complete its work in 2015 or so.
Ultimately, a great deal depends on precisely where the Higgs boson is found. If it’s below 120 GeV then you simply need a muon collider if you want precision measurements. As we move above 120GeV, and especially above 130GeV, the charged vector boson decay process rapidly takes over from the s-channel as the decay width expands dramatically. Then traditional linear collider alternatives such as CLIC become competitive. After all, they have two orders of magnitude higher luminosity than a muon collider and there is less technological uncertainty associated with them, and since the Higgs width is now much wider, and the s-channel no longer dominant, the unique precision of the muon collider is less relevant.
But in any event, the recent exclusion of Higgs above 145GeV has already done serious damage to the rationale and justification for CLIC and will do the muon collider case no harm at all, assuming, that is, that we find the Higgs.

With the benefit of hindsight this is now known to be due to production of the J/psi. The BNL management wanted someone to propose an experiment to investigate the Lederman shoulder, but years went by. In the meantime Lederman went to CERN and ran an experiment at the ISR, but so much `background noise’ was coming from the mass range of 3.1 GeV/c^2 that they cut it out. Also at the ADONE ring at Frascati, Italy, they found background noise from the mass range of 3.1 GeV/c^2 and they cut it out. At the same time the CEA (Cambridge Electron Accelerator ~ MIT/Harvard) reported an increase in R (ratio of e+e- –> hadrons/e+e- –> mu+mu-) consistent with the creation of a new quark. But nobody believed CEA because it had lost credibility because of prior publication of false claims. Finally Sam Ting proposed an experiment (at the AGS at BNL) which did the job. (Although Ting did not specifically propose his experiment to investigate the Lederman shoulder.) Ting’s experiment was sufficiently detailed that it proved the existence of a new particle of width less than 10 MeV/c^2 (the J). Then it becomes the well-known story of how Ting delayed publication, and Richter’s team at SPEAR (using the Mark II detector) discovered the psi, and there was no doubt of the existence of a narrow resonance, with width less than 1 MeV/c^2 (the psi). It was only after that, that the various puzzling results were recognized as the J/psi.

So one hopes that there is not a similar glossing over of backgrounds (or false assumptions about what constitutes `noise’) or whatever, especially given that this time around people are specifically searching for the Higgs.

To expand on what VP mentionned, what I’ve noticed is that:
1) last june there has been such an interim report (see this page ) presented as “Given the level of public investment and interest in the LHC and implications of the LHC’s discovery potential for the natural and human science fields, the Council underlined the importance of a policy on communicating discoveries at the LHC that was geared to providing information accessible to politicians, the general public and other scientific disciplines and not just to the particle physics community”
2) the final report will apparently be delivered on september 15 by the Chairman of the Scientific Policy Committee (see http://indico.cern.ch/conferenceDisplay.py?confId=152955 ).

But not sure what to make of that: that’s his job after all to plan for every possible outcome, just in case, not necessarily anything to do with as yet undisclosed data.

DB, thanks for the fascinating info on muon colliders. Am I correct in thinking that “gluon-gluon fusion” here is a typo for “muon-antimuon fusion” via the Yukawa coupling? The idea being that you would tune the beam energy to the Higgs peak, as they did with the Z peak at LEP2?

@DB, perhaps the muon collider, and also all the SUSY scenarios involve speculation too… in the old days one had to measure a particle’s mass, spin, width/lifetime, parity, and C (when pertinent) to declare it a particle. It seems a bit less than open ended to work out Higgs limit plots with a variable width, but that is just my opinion.

@jpsi, great stuff, great stories. There are so, so many with the J/Psi. Let’s pray for a rerun with something new!

@Chris
For a suite of Feynman diagrams that show the various ways muon-antimuon interactions can generate Higgs see the Higgs/Scalar Physics section ofhttp://home.fnal.gov/~rruiz/FeynLib/

You’re correct, in a Higgs Factory, muon collider centre-of-mass energy is set equal to the Higgs mass. Then this centre-of-mass energy is varied over a narrow range so as to scan over the Higgs resonance. One of the benefits of LEP2 having excluded the Higgsbottom anti-bottom pairs which would have compromised a muon collider’s sensitivity to Higgs. So if there is a Higgs around 120 GeV, then a muon collider is really in the sweet spot as far as not only precisely determining Higgs parameters, but also in distinguishing between SM and SUSY versions of Higgs.

One further point about Peter’s neutrino shielding issues: Using a conventional accelerator as a beam source of neutrinos, as originally proposed in the late fifties by Pontecorvo and Schwarz and implemented at CERN in the early sixties, does require massive shielding, because here you smash protons into a target, generating a large number of pions which have to then decay over a distance into muons and neutrinos. Similarly if you are using a nuclear reactor as a neutrino source where you are restricted to antineutrinos and cannot generate collimated beams. But muons just decay into electrons/positrons which are easy to shield.

That should have read: One of the benefits of LEP2 having excluded the Higgs below115GeV is that there is very significant background at the Z-pole via Z–>bottom anti-bottom pairs which would have compromised a muon collider’s sensitivity to Higgs.

What I was referring to is a different potential problem, the intensity of the neutrino beam from the muon decays being high enough to create an unshieldable radiation hazard off-site, see for instance here:

It should be made clear that those articles are about the idea of using gauge symmetry and spontaneous symmetry breakdown in GR, and this, until somebody has a really good idea about unifying quantum gravity and the SM, has nothing to do with gauge symmetry and spontaneous symmetry breakdown in the the electroweak theory, which is what the LHC results are relevant to.

To be able to examine a 120GeV Higgs with a muon collider, the Fermilab studies concluded one would only need a centre of mass energy sqrt(s) in the range 100-500GeV, commonly known as the FMC or First Muon Collider. The paper you cite only considers high energy muon colliders in the 1-4TeV range where the neutrino radiation begins to become a serious concern. However, even the authors conclude “For CoM energies exceeding 4 TeV (my emphasis) some countermeasure must be adopted to limit the radiation dose”. Furthermore it takes no account of muon ionization cooling which would be expected to contribute significantly to reduced muon decay and lowered neutrino radiation levels. Incidentally a 4TeV version is considered the ideal machine to fully explore strong vector boson scattering – a far cry from the 40TeV estimate required for the SSC.

In any case, it’s difficult to imagine building a 4TeV collider without having prototyped the various novel technologies with an FMC in the first place.

I’ll just recast that table:
Exclusion of SM Higgs: Excluding an SM Higgs Boson at 95% confidence level down to 114 GeV requires significantly less data than discovering same to 5 sigma over the same range. As I’m unsure as to whether Ruiz’s numbers are correct for the 95% confidence exclusion I won’t quote them here but in any event both LHC and Tevatron should have enough data already to do this.

Discovery of SM Higgs: For a 5 sigma detection of a SM Higgs based on a joint analysis of Atlas and CMS data their simulations project that they need the following:
Down to 140GeV: 1 fb-1 for each experiment separately and subsequently combined
Down to 128GeV: 2.5 fb-1 ditto
Down to 117Gev: 5 fb-1 ditto
Down to 114GeV: 7.5 fb-1 ditto

On June 17th the LHC had collected 1 fb-1 for each experiment.On August 5th, 2 fb-1. They expect to achieve 5 fb-1 by end October which, when combined (joint analysis), tells us that by then they should have accumulated enough data to do the discovery job down to 117GeV. End 2011 for a preliminary result one way or the other is quite realistic.

The reason we are hearing from the Tevatron now is that, although they may not be in a position to discover a SM Higgs boson to 5 sigma levels, they have enough data to exclude one at 95% confidence level.

It will be quite a bitter pill for CERN to swallow if Fermilab gets to the exclusion finish line first.

@null: “If there is no Higgs, then does the 10^55 cosmological constant/energy density no longer a problem (i.e QFT calculation for particle physics for cc can be close to the astronomical cc)”

I don’t think so. You still have to somehow resolve the problem of the vacuum energy of all the other fields in the Standard Model. Formally, the contribution of each field to the vacuum energy is infinite, although these can typically be renormalized to pretty much any value you like by what amounts to a simple redefinition of the measure of the path integral. The signs of the vacuum contributions may be different for different fields, but it is difficult to make an argument that they would all cancel (never mind “almost” cancel, which is probably even more difficult) without some symmetry principle, lacking in the Standard Model.

I think the mood in the HEP community as a whole is very positive, with a lot of excitement that the answer to the question “is there a SM Higgs?” may finally be close.

The mood among SUSY enthusiasts may not be so good (they’re still in the first stage of grief, denial). As for the mood among those who seriously thought the LHC would see extra dimensions, that also would not be good, but I’m not sure there’s more than a vanishingly small number of such people.

“One further point about Peter’s neutrino shielding issues: Using a conventional accelerator as a beam source of neutrinos, as originally proposed in the late fifties by Pontecorvo and Schwarz and implemented at CERN in the early sixties,….”

Actually the first accelerator source of neutrinos for an experiment was used by Mel Schawartz himself (along with Leon Lederman, Jack Steinberger, et al) in the early sixties at the AGS at Brookhaven and not CERN. It led to the discovery of the muon neutrino and a Nobel prize for the three mentioned above a quarter century later. As Jack once advised: “Do your good work early and live long enough”.

@null
Professor Guido Tonelli, the spokesman for CMS, told the BBC on Sep.1st: “We could discover the Standard Model version of the Higgs Boson or exclude it earlier than expected. Could we discover it by Christmas? In principle, yes,”

In fact, both Fermilab and the LHC should be in a position to exclude the Higgs by Christmas, if it doesn’t exist. If it exists, discovery to 5 sigma will take longer and only the LHC has a realistic chance of achieving this. The closer the Higgs is to 114 GeV, the harder it is for the LHC to see it and the longer it will take.

@VP
I was wondering if anyone was awake. 🙂 Actually CERN was ready to go before Brookhaven, but an outsider, Guy von Dardel was brought in to verify the neutrino flux calculations and he argued that the team (wrongly it turns out) had made important errors, as a result CERN postponed startup until June 1963, too late to do anything but verify the Brookhaven results.

There is signal above background at various places in this mass range, just not enough give a statistically significant signal. And, with the amount of data available, you shouldn’t see a statistically significant signal (at least not in the lower part of this range). So, for the moment, all is consistent with either Higgs or no Higgs in this region. It will take more data to resolve this, everyone’s waiting…